The invention relates to a martensitic steel alloy and more particularly to such an alloy as material for fuel cladding for liquid metal cooled reactors.
One of the prime objectives in the efforts to develop a commercially viable liquid metal fast breeder reactor (LMFBR) has been to develop an alloy, or alloys, which are swelling resistant and have the required inreactor mechanical properties for use as fuel cladding and/or use as ducts. The fuel cladding will see service in contact with flowing liquid sodium and have a surface temperature of about 400.degree. C. (.about.715.degree. F.) to 650.degree. C. (.about.1200.degree. F.). A duct surrounds each bundle of fuel pins and sees service at about 380.degree. C. (.about.715.degree. F.) to 550.degree. C. (.about.1020.degree. F.). These components will be exposed at the aforementioned elevated temperatures to fast neutron fluxes on the order of 10.sup.15 n/cm.sup.2 .multidot.S (E&gt;0.1 MeV), and should be capable of performing adequately to fluences on the order of 2 to 3.times.10.sup.23 n/cm.sup.2 (E&gt;0.1 MeV).
Initially, one of the prime candidate alloys for commercial LMFBR, fuel cladding and ducts was 20% cold worked AISI 316 steel, a solid solution strengthened austenitic stainless steel (see Bennett and Horton, "Material Requirements for Liquid Metal Fast Breeder Reactor," Metallurgical Transactions A, Vol. 9A, February 1978, pp. 143-149). Typical chemistry and material fabrication steps for nuclear grade 316 fuel cladding are described in U.S. Pat. No. 4,421,572 filed on Mar. 18, 1982. The specification of U.S. Pat. No. 4,421,572 is hereby incorporated by reference.
Current commercial composition specifications for nuclear grade 316 stainless steel contain only a maximum value for impurities such as phosphorus, sulphur, boron, aluminum, niobium, vanadium, tantalum, copper and cobalt. Typical commercial melting procedure for this alloy involves double-vacuum melting of electrolytic-grade starting materials. This practice results in low levels of the aforementioned impurities, which depending on the particular impurity, may be 10 to 100 times less than the maximum value allowed by the specification.
However, the 316 alloy undergoes a high degree of void swelling during extended exposure to fast neutron fluxes at the LMFBR operating temperatures. Extensive development efforts aimed at reducing swelling have been undertaken, and are exemplified by U.S. Pat. No. 4,158,606 and U.S. Pat. No. 4,407,673 filed on Jan. 9, 1980. U.S. Pat. No. 4,576,641 pertains to austenitic stainless steels containing increased levels of phosphorus to provide enhanced inreactor swelling resistance. U.S. Pat. No. 4,530,719 provides a solid solution strengthened austenitic stainless steel and notes that stress rupture strength increases as the sum of the phosphorus, sulphur and boron contents of the alloy increase. While the aforementioned efforts have led to improvements in swelling resistance, the stress rupture behavior of these alloys in fuel pin cladding applications remains as one of the major limitations on fuel pin life and improvements in this area are needed for long-life LMFBR cores.